So you surf the internet , see a clip or movie that you can download and you download it .
Then you attempt to play the file and your computer stops , then you receive a message to play this you will need to go on the internet to find the correct codec ( or words to that effect ).
This is where you should STOP.
You are probably about to be infected by spyware or a virus if you agree to download or search for the codec.
Your best bet is to get the VLC Player which can play most video files , dvd’s , cd’s , streaming media , check out the site for the full list.
If VLC cannot play it , just delete the file and run a full virus / spyware scan.
Alternatively download the K Lite Codec pack for Windows which should allow most video files to be played in Windows Media Player.